How to make a “reducer enhancer” for Angular ngrx store for undo / redo

Question

I would like to add a reducer enhancer or meta reducer for a specific store slice. I've implemented a function that takes in a reducer and enables undo / redo functionality. It currently looks like this:

export const adminReducers: ActionReducerMap<AdminFeatureState> = {
    admin: admin.reducer,
    dynamicForm: undoable(dynamicForm.reducer, DynamicFormActionTypes.UNDO, DynamicFormActionTypes.REDO, [
        DynamicFormActionTypes.SELECTED_DYNAMIC_CONTROLS_CHANGED,
        DynamicFormActionTypes.CHANGE_GROUP,
        DynamicFormActionTypes.RESET,
        DynamicFormActionTypes.ADD_PAGE,
        DynamicFormActionTypes.REMOVE_PAGE,
        DynamicFormActionTypes.REORDER_GROUPS,
        DynamicFormActionTypes.SAVE_EDIT_CONTROL
    ])
};

export function undoable<T>(reducer: ActionReducer<any>, undoActionType: string, redoActionType: string, recordedActions: string[]): ActionReducer<any> {
    // Call the reducer with empty action to populate the initial state
    const initialState: UndoableState<T> = {
        past: [],
        present: reducer(undefined, { type: 'INIT' }),
        future: []
    };

    // Return a reducer that handles undo and redo
    return function (state = initialState, action) {
    ...
    };
}

Everything works great except when I build for production I get the following error:

Error during template compile of 'AdminModule'
  Function calls are not supported in decorators but 'undoable' was called in 'adminReducers'
    'adminReducers' calls 'undoable' at src\app\core\containers\admin\admin-feature.reducers.ts(11,29).

The only other way I have seen you can enhance an existing reducer is using meta-reducers but those get called for every reducer function, not just a specific one such as the 'dynamicForm' in this case.

Answer 1

After some digging in the docs I found the solution here:

https://github.com/ngrx/platform/blob/master/docs/store/api.md#feature-module-reducers-and-aot

You can simply wrap 'adminReducers' in combineReducers like so:

const adminReducers: ActionReducerMap<AdminFeatureState> = {
    admin: admin.reducer,
    dynamicForm: undoable(dynamicForm.reducer, DynamicFormActionTypes.UNDO, DynamicFormActionTypes.REDO, [
        DynamicFormActionTypes.SELECTED_DYNAMIC_CONTROLS_CHANGED,
        DynamicFormActionTypes.CHANGE_GROUP,
        DynamicFormActionTypes.RESET,
        DynamicFormActionTypes.ADD_PAGE,
        DynamicFormActionTypes.REMOVE_PAGE,
        DynamicFormActionTypes.REORDER_GROUPS,
        DynamicFormActionTypes.SAVE_EDIT_CONTROL
    ])
};

const adminMetaReducer = combineReducers(adminReducers);

export function enhancedAdminReducers(state, action) {
    return adminMetaReducer(state, action);
}

Then import that into your module

StoreModule.forFeature(ADMIN_FEATURE, enhancedAdminReducers),